1. Field of the Invention
The present invention relates to a radiation image conversion panel using a stimulable phosphor.
2. Description of the Related Art
As a method which can replace conventional radiography, a radiation image conversion method using a stimulable phosphor is described, for example, in Japanese Patent Application Laid-Open (JP-A) No. 55-12145. This method utilizes a radiation image conversion panel containing a stimulable phosphor (also referred to as an accumulative phosphor sheet). In this method, radiation transmitted through a subject or emitted from an object to be examined is absorbed into the stimulable phosphor of the above-mentioned panel, and the stimulable phosphor is excited by an electromagnetic wave (stimulating ray) such as visible light or infrared light in a time sequential manner so as to allow radiation energy accumulated in the stimulable phosphor to emit as fluorescent emission (stimulated emission). Subsequently, an electric signal is obtained by photoelectrically reading the fluorescence, and based on the obtained electric signal, a radiation image of the subject or object to be examined is reproduced as a visible image.
The above-described radiation image conversion method provides an advantage that a radiation image having abundant information can be obtained by a radiation dosage far smaller than that in conventional radiography using a combination of a radiographic film and sensitized paper. Accordingly, this method is of great value of utilization particularly in direct medical radiography such as X-ray photography used for diagnostic purposes.
The radiation image conversion panel used in the radiation image conversion method has a basic structure comprised of a support and a phosphor layer provided on one side of the support. However, the support is not necessarily needed if the phosphor layer is a self-supporting layer. Further, the surface of the phosphor layer opposite to the side at which the support is located (i.e., the surface of the phosphor layer which does not face the support) has generally a transparent protective layer to thereby protect the phosphor layer from chemical deterioration or physical impact.
Generally, the phosphor layer is comprised of a stimulable phosphor and a binder which contains and supports the phosphor in a state of dispersion. When the stimulable phosphor is irradiated with stimulating ray after having absorbed radiation such as X-rays, it exhibits the property of stimulated emission. Accordingly, the radiation transmitted through a subject or emitted from an object to be examined is absorbed in the phosphor layer of the radiation image conversion panel in proportion to the amount of the radiation, and a radiation image of the subject or the object to be examined is formed as an image of accumulated radiation energy in the radiation image conversion panel. The accumulation image can be released as stimulated emission light by the irradiation with stimulating ray, and photoelectrically reading the stimulated emission light and converting the same into an electric signal, so that the radiation energy accumulation image can be made into a visible image.
The radiation image conversion method is, as described above, a greatly advantageous image formation method. A radiation image conversion panel used by the radiation image conversion method is desirably one having a high sensitivity and providing an image of excellent image quality (sharpness, graininess, and the like) as well as sensitized paper used by conventional radiography. The sensitivity of the radiation image conversion panel basically depends on a total quantity of stimulated emission light of a stimulable phosphor contained in the radiation image conversion panel, and the total quantity of emission light varies not only due to light emission intensity of the phosphor itself, but also due to an amount of the stimulable phosphor contained in the phosphor layer. If the amount of the stimulable phosphor contained is large, the phosphor layer absorbs a large amount of radiation such as X-rays. Therefore, the sensitivity of the panel can be made higher, and simultaneously, the image quality (particularly, graininess) of the panel improves. On the other hand, when the amount of the stimulable phosphor contained in the phosphor layer is constant, a layer thickness of stimulable phosphor grains can be made thinner as packing density of the phosphor grains becomes greater. As a result, spread of stimulating ray caused by scattering can be reduced and relatively better sharpness can be obtained.
There has recently been increased demand for improved image quality, and therefore, a radiation image conversion panel obtained by forming a phosphor layer on a support and by compressing the phosphor layer is disclosed in JP-A Nos. 59-126299 and 59-126300. The radiation image conversion panel thus obtained allows, by compression processing of the phosphor layer, density of a stimulable phosphor in the phosphor layer to become greater than that in conventional radiation image conversion panels.